Amplification by PCR has sequence-dependence efficiency, especially the sequence that binds to primers. If one sequence is amplified 10% more than another in one round, it will be 1.1³⁰ = 17.4 x more abundant after 30 rounds. This effect is most important when the sequence has one or more mismatches with the primer. With one mismatch, amplification efficiency is usually significantly less, and with two or more mismatches the sequence may not be amplified to detectable levels.